Dynamic Equilibrium
Chemical equilibrium is a dynamic state in which the rate of the forward reaction equals the rate of the reverse reaction. At equilibrium, concentrations of reactants and products remain constant (not necessarily equal). The system appears static macroscopically but reactions continue at the molecular level.
Equilibrium Constant (Kc)
Le Chatelier's Principle
When an equilibrium is disturbed, the system shifts to partially counteract the disturbance. Increasing reactant concentration → equilibrium shifts right (more products). Increasing pressure in a gas-phase reaction → shifts toward fewer moles of gas. Increasing temperature → shifts toward the endothermic direction.
Effect of Changes on Equilibrium
Le Chatelier's principle applied to common disturbances.
| Disturbance | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| ↑ Reactant conc. | Shift right (→) | Shift right (→) |
| ↑ Product conc. | Shift left (←) | Shift left (←) |
| ↑ Pressure | Shift to fewer gas moles | Shift to fewer gas moles |
| ↑ Temperature | Shift left (←) — K decreases | Shift right (→) — K increases |
| Add catalyst | No shift — reaches equilibrium faster | Same |
HSC Exam Focus
Kc > 1 means products are favoured at equilibrium. Kc < 1 means reactants are favoured. A catalyst does NOT change Kc or shift equilibrium — it only speeds up both forward and reverse reactions equally.
Biochemistry Bridge
Haemoglobin's binding of oxygen is an equilibrium reaction: Hb + O₂ ⇌ HbO₂. In the lungs (high [O₂]), the equilibrium shifts right — haemoglobin loads O₂. In tissues (low [O₂] and higher CO₂), it shifts left — O₂ is released. Le Chatelier's principle keeps you alive.
InstaTest
InstaTest: Equilibrium
MCQ questions on Kc, Le Chatelier's principle, and equilibrium shifts.